Self-propelled work machines, such as trucks, wheeled tractors, track type tractors and other construction vehicles, are used for a variety of tasks. These machines must be able to work efficiently over a variety of ground conditions, such as subsoil and tilled and untilled earth. In a work machine, one measure of operating efficiency, termed the “tractive efficiency,” is defined as the ratio of the drawbar power (i.e., the power available to push or pull things with the tractor) to the power of the machine's motor. The drawbar power is a function of the drawbar force, which is, in turn, dependent upon the difference between the wheel or track speed, i.e., the theoretical speed (Vt) of the work machine, and the actual speed (Vact) of the machine over the ground. This difference is termed “slip,” as it is due to the wheels or tracks slipping along the ground surface.
A condition in which no net drawbar force is generated may correspond to a condition with zero slip (Vact=Vt). If the machine is operating with zero slip, then all of the motor power is being used to overcome ground friction and to maintain current machine speed. Consequently, no net drawbar force is generated. A certain amount of slip accompanies the generation of drawbar force, and the amount of slip increases as drawbar force increases. However, the relationship between drawbar force and slip is not linear. As drawbar force is increased, each successive unit increase in drawbar force is accompanied by a progressively larger increase in slip. Ultimately, a maximum drawbar force is obtained. At this limit, further attempts to increase drawbar force result only in additional slip and no increase in drawbar force. This additional slip results in unnecessary fuel consumption and wear on the machine. Thus, controlling slip may be desired in order to operate a work machine efficiently.
Prior art systems for controlling slip in tractors including conventional propulsion systems have been developed. One such system is described in U.S. Pat. No. 5,287,280, entitled “Method and Apparatus for Controlling Shoe Slip of Crawler Vehicle.” In this system, a shoe slip ratio is calculated from the difference between the traveling speed of the vehicle and the traveling speed of its tracks. When the shoe slip ratio exceeds a preset value, the power output of an internal combustion engine is cut off by a fuel injection governor. However, this technique of power control is not applicable to machines having electric drive propulsion systems. Further, the system disclosed in the '280 patent is not adjustable to optimize slip for different ground conditions or to allow an operator to select a desired tractive efficiency for current operations.
The presently disclosed slip control system is directed to solving one or more of these shortcomings of the prior art slip control systems.